The production of high quality articles, particularly photographic, photothermographic, and thermographic articles, consists of applying a thin film of a coating solution on to a continuously moving substrate or web. Thin films can be applied using a variety of techniques including: dip coating, forward and reverse roll coating, wire wound rod coating, blade coating, slot coating, slide coating, and curtain coating. Coatings can be applied as a single layer or as two or more superimposed layers. Although it is usually most convenient for the substrate to be in the form of a continuous web, it may also be formed of a succession of discreet sheets.
Slide coaters have been used extensively since the 1950's in the photographic and related industries for coating aqueous photographic emulsions with relatively low viscosity (less than 100 cP). In slide coating, it is well known to start and stop coating of a moving web by means known as "pick-up." In the pick-up phase, the flow of the coating liquid is established with the coater die retracted from the web. The coating liquid drains over the die edge into a vacuum box and drain. Once the flows of all the coating liquids are stabilized from all the feed slots of the slide coating die, the die and vacuum box are moved into the coating position in a rapid manner with the web moving at the desired coating speed. When the die is in close proximity to the moving web, the coating liquid forms a coating bead that coats the web rather than draining over the die edge. If the coating process needs to be interrupted (for example, as a web splice is passing in the slide coating die), the die and vacuum box assembly are simply retracted from the web until resumption of the coating is desired.
Streak-type defects can be formed by disturbances of the coating bead. Mechanical disturbances include nicks in the die edge. Contamination disturbances that may cause streaking include dirt particles lodged near the coating bead, dried or semi-dried particles of coating compound, and non-uniform wetting of the contact line of the coating liquid on the coating die edge. Non-uniform wetting on the die edge, especially after pick-up, appears to be an important factor when coating fluids containing volatile solvents. For example, contamination may adhere to the front face and/or die edge of the slide coating die. That contamination may lead to a non-uniform wetting line and possible streaking of the coating compound.
FIG. 1 illustrates an exemplary slide coating die 20 in which a coating fluid 22 is flowing along a slide surface 24 to a die edge 26. A static wetting line 28 is formed along a front face 30 of the slide coating die 20. The irregular shape of the static wetting line 28 is likely to cause unevenness and streaking of the coating fluid as it is applied to the moving web (not shown).
Another problem related to slide coating is contamination of vacuum ports and drains in the vacuum box when the die is retracted from the moving web and the coating liquid is flowing freely. Contamination of the vacuum ports and drains can lead to unstable vacuum operation causing defects and eventually requiring cessation of the coating operation to clean the vacuum box and ports. This problem is exacerbated with high viscosity fluids 100-10,000 cP) that contain volatile solvents that dry much faster than water (such as methyl ethyl ketone, tetrahydrofuran, or methanol).